---
title: 'Measuring Code Coverage in Cypress'
description: 'Learn how to measure code coverage in Cypress and the difference between UI Coverage and Code Coverage.'
sidebar_label: 'Code Coverage'
---

<ProductHeading product="app" />

# Measure Code Coverage in Cypress

:::info

##### <Icon name="question-circle" color="#4BBFD2" /> What you'll learn

- The difference between Code Coverage and Cypress's UI Coverage
- How to instrument your application code for code coverage
- How to collect code coverage from Cypress tests
- How to merge code coverage from parallel tests
- How to collect code coverage from different testing types

:::

## Introduction

As you write more and more end-to-end tests, you will find yourself wondering -
do I need to write more tests? Are there parts of the application still
untested? Are there parts of the application that perhaps are tested too much?

Cypress offers a couple of solutions to help answer these questions:

- **[Code coverage](#Code-Coverage)** - find out which lines of the application's source code were executed
- **[UI coverage](#UI-Coverage)** - find out which parts of the UI were tested

### Code Coverage

Code coverage is a metric that helps you understand how much of your application
code is covered by your tests. If there are important
sections of the application's logic that **were not** executed from the tests,
then a new test can be added to ensure that the application logic for these sections is
also tested.

Computing the source code lines that were executed during the test is done
through **code coverage**. Code coverage requires inserting additional counters
into your source code before running it using **instrumentation**.
This document will be focused on code coverage and the instrumentation required to set it up.

### UI Coverage

UI Coverage is a visual test coverage report based on the interactive elements that make up your application.
It highlights areas of missed test coverage directly within every page of your application,
that you can use to make data-driven testing decisions.

UI Coverage also seamlessly integrates with your Cypress tests, building on the tests you've already
written — no extra instrumentation is required like with code coverage.

To learn more about UI coverage,
please see our [UI Coverage](/ui-coverage/get-started/introduction) guide.

<DocsImage
  src="/img/ui-coverage/get-started/uicov-docs-1.gif"
  alt="UI Coverage demo showing UI of Cloud product"
/>

## Instrumenting code

<DocsVideo
  src="https://youtube.com/embed/C8g5X4vCZJA"
  title="Complete Code Coverage with Cypress"
/>

Instrumentation takes code that looks like this...

```javascript title='add.js'
function add(a, b) {
  return a + b
}
module.exports = { add }
```

...and parses it to find all functions, statements, and branches and then
inserts **counters** into the code. For the above code it might look like this:

```javascript
// this object counts the number of times each
// function and each statement is executed
const c = (window.__coverage__ = {
  // "f" counts the number of times each function is called
  // we only have a single function in the source code thus it starts with [0]
  f: [0],
  // "s" counts the number of times each statement is called
  // we have 3 statements and they all start with 0
  s: [0, 0, 0],
})

// the original code + increment statements
// uses "c" alias to "window.__coverage__" object
// the first statement defines the function, let's increment it
c.s[0]++
function add(a, b) {
  // function is called and then the 2nd statement
  c.f[0]++
  c.s[1]++

  return a + b
}
// 3rd statement is about to be called
c.s[2]++
module.exports = { add }
```

Imagine we load the above instrumented source file from our test spec file.
Immediately some counters will be incremented!

```javascript title='add.cy.js'
const { add } = require('./add')
// JavaScript engine has parsed and evaluated "add.js" source code
// which ran some of the increment statements
// __coverage__ has now
// f: [0] - function "add" was NOT executed
// s: [1, 0, 1] - first and third counters were incremented
// but the statement inside function "add" was NOT executed
```

We want to make sure every statement and function in the file `add.js` has been
executed by our tests at least once. Thus we write a test:

```javascript title='add.cy.js'
const { add } = require('./add')

it('adds numbers', () => {
  expect(add(2, 3)).to.equal(5)
})
```

When the test calls `add(2, 3)`, the counter increments inside the "add"
function are executed, and the coverage object becomes:

```javascript
{
  // "f" keeps count of times each function was called
  // we only have a single function in the source code
  // thus it starts with [0]
  f: [1],
  // "s" keeps count of times each statement was called
  // we have 3 statements, and they all start with 0
  s: [1, 1, 1]
}
```

This single test has achieved 100% code coverage - every function and every
statement has been executed at least once. But, in real world applications,
achieving 100% code coverage requires multiple tests.

Once the tests finish, the coverage object can be serialized and saved to disk
so that a human-friendly report can be generated. The collected coverage
information can also be sent to external services and can help during pull request
reviews.

:::info

If you are unfamiliar with code coverage or want to learn more, take a look at
the "Understanding JavaScript Code Coverage" blog post
[Part 1](https://medium.com/engineering-semantics3/understanding-code-coverage-1074e8fccce0/)
and
[Part 2](https://medium.com/engineering-semantics3/understanding-javascript-code-coverage-part-2-9aedaa5119e5/).

:::

Cypress does not instrument your code - you need to do it yourself. The golden
standard for JavaScript code instrumentation is the battle-hardened
[Istanbul](https://istanbul.js.org) and, luckily, it plays very nicely with Cypress.
You can instrument the code as a build step through one of two ways:

- Using the [nyc](https://github.com/istanbuljs/nyc) module - a command-line
  interface for the [Istanbul](https://istanbul.js.org) library
- As part of your code transpilation pipeline using the
  [`babel-plugin-istanbul`](https://github.com/istanbuljs/babel-plugin-istanbul)
  tool

### Using NYC

To instrument the application code located in your `src` folder and save it in
an `instrumented` folder use the following command:

```shell
npx nyc instrument --compact=false src instrumented
```

We are passing the `--compact=false` flag to generate human-friendly output.

The instrumentation takes your original code like this fragment...

```js title='src/index.js'
const store = createStore(reducer)

render(
  <Provider store={store}>
    <App />
  </Provider>,
  document.getElementById('root')
)
```

...and wraps each statement with additional counters that keep track of how many
times each source line has been executed by the JavaScript runtime.

```javascript
const store = (cov_18hmhptych.s[0]++, createStore(reducer))
cov_18hmhptych.s[1]++
render(
  <Provider store={store}>
    <App />
  </Provider>,
  document.getElementById('root')
)
```

Notice the calls to `cov_18hmhptych.s[0]++` and `cov_18hmhptych.s[1]++` that
increment the statement counters. All counters and additional book-keeping
information are stored in a single object attached to the browser's `window`
object. We can see the counters if we serve the `instrumented` folder instead of
`src` and open the application.

<DocsImage
  src="/img/app/code-coverage/coverage-object.png"
  alt="Code coverage object"
/>

If we drill into the coverage object we can see the statements executed in each
file. For example the file `src/index.js` has the following information:

<DocsImage
  src="/img/app/code-coverage/coverage-statements.png"
  alt="Covered statements counters in a from the index file"
/>

In green, we highlighted the 4 statements present in that file. The first three
statements were each executed once and the last statement was never executed (it
probably was inside an `if` statement). By using the application, we can both
increment the counters and flip some of the zero counters into positive numbers.

### Using code transpilation pipeline

Instead of using the `npx instrument` command, we can use
[`babel-plugin-istanbul`](https://github.com/istanbuljs/babel-plugin-istanbul)
to instrument the code as part of its transpilation. Add this plugin to the
`.babelrc` file.

```json title='.babelrc'"
{
  "presets": ["@babel/preset-react"],
  "plugins": ["transform-class-properties", "istanbul"]
}
```

We can now serve the application and get instrumented code without an
intermediate folder, but the result is the same instrumented code loaded by the
browser, with the same `window.__coverage__` object keeping track of the
original statements.

:::info

Check out
[`@cypress/code-coverage#examples`](https://github.com/cypress-io/code-coverage#examples)
for full example projects showing different code coverage setups.

:::

<DocsImage
  src="/img/app/code-coverage/source-map.png"
  alt="Bundled code and source mapped originals"
/>

A really nice feature of both [nyc](https://github.com/istanbuljs/nyc) and
[`babel-plugin-istanbul`](https://github.com/istanbuljs/babel-plugin-istanbul)
is that the source maps are generated automatically, allowing us to collect code
coverage information, but also interact with the original, non-instrumented code
in the Developer Tools. In the screenshot above, the bundle (green arrow) has
coverage counters, but the source mapped files in the green rectangle show the
original code.

:::info

The `nyc` and `babel-plugin-istanbul` only instrument the application code and
not 3rd party dependencies from `node_modules`.

:::

## E2E code coverage

To handle code coverage collected during each test, there is a
[`@cypress/code-coverage`](https://github.com/cypress-io/code-coverage) Cypress
plugin. It merges coverage from each test and saves the combined result. It also
calls `nyc` (its peer dependency) to generate static HTML reports for human
consumption.

### Install the plugin

:::info

Please consult the
[`@cypress/code-coverage`](https://github.com/cypress-io/code-coverage)
documentation for up-to-date installation instructions.

:::

```shell
npm install @cypress/code-coverage --save-dev
```

Then add the code below to the
[supportFile](/app/references/configuration#e2e) and
[setupNodeEvents](/app/plugins/plugins-guide#Using-a-plugin) function.

```js
// cypress/support/e2e.js
import '@cypress/code-coverage/support'
```

:::cypress-config-plugin-example

```js
require('@cypress/code-coverage/task')(on, config)
// include any other plugin code...

// It's IMPORTANT to return the config object
// with any changed environment variables
return config
```

:::

When you run the Cypress tests now, you should see a few commands after the
tests finish. We have highlighted these commands using a green rectangle below.

<DocsImage
  src="/img/app/code-coverage/coverage-plugin-commands.png"
  alt="coverage plugin commands"
/>

After the tests complete, the final code coverage is saved to a `.nyc_output`
folder. It is a JSON file from which we can generate a report in a variety of
formats. The
[`@cypress/code-coverage`](https://github.com/cypress-io/code-coverage) plugin
generates the HTML report automatically - you can open the `coverage/index.html`
page locally after the tests finish. You can also call `nyc report` to generate
other reports, for example, sending the coverage information to 3rd party
services.

### See code coverage summary

To see the summary of the code coverage after tests run, run the command below.

```shell
npx nyc report --reporter=text-summary

========= Coverage summary =======
Statements   : 76.3% ( 103/135 )
Branches     : 65.31% ( 32/49 )
Functions    : 64% ( 32/50 )
Lines        : 81.42% ( 92/113 )
==================================
```

:::info

**Tip:** store the `coverage` folder as a build artifact on your continuous
integration server. Because the report is a static HTML page, some CIs can show
it right from their web applications. The screenshot below shows the coverage
report stored on CircleCI. Clicking on `index.html` shows the report right in
the browser.

:::

<DocsImage
  src="/img/app/code-coverage/circleci-coverage-report.png"
  alt="coverage HTML report on CircleCI"
/>

## Code coverage as a guide

Even a single test can cover a lot of the application code. For example, let's
run the following test that adds a few items, then marks one of them as
completed.

:::visit-mount-example

```js
it('adds and completes todos', () => {
  -{cy.visit('/')::cy.mount(<AddTodo />)}-
  cy.get('.new-todo').type('write code{enter}')
  cy.get('.new-todo').type('write tests{enter}')
  cy.get('.new-todo').type('deploy{enter}')

  cy.get('.todo').should('have.length', 3)

  cy.get('.todo').first().find('.toggle').check()

  cy.get('.todo').first().should('have.class', 'completed')
})
```

:::

After running the test and opening the HTML report, we see 76% code coverage in
our application.

<DocsImage
  src="/img/app/code-coverage/single-test.png"
  alt="Coverage report after a single test"
/>

Even better, we can drill down into the individual source files to see what code
we missed. In our example application, the main state logic is in the
`src/reducers/todos.js` file. Let's see the code coverage in this file:

<DocsImage
  src="/img/app/code-coverage/todos-coverage.png"
  alt="Main application logic coverage"
/>

Notice how the **ADD_TODO** action was executed 3 times - because our test has
added 3 todo items, and the **COMPLETE_TODO** action was executed just once -
because our test has marked 1 todo item as completed.

The source lines not covered, marked in yellow (the switch cases the test missed)
and red (regular statements) are a great guide for writing more end-to-end
tests. We need tests that delete todo items, edit them, mark all of them as
completed at once and clear completed items. When we cover every switch
statement in `src/reducers/todos.js` we probably will achieve close to 100% code
coverage. Even more importantly, we will cover the main features of the
application the user is expected to use.

We can write more E2E tests.

<DocsImage
  src="/img/app/code-coverage/more-tests.png"
  alt="Cypress passed more tests"
/>

The produced HTML report shows 99% code coverage

<DocsImage
  src="/img/app/code-coverage/almost-100.png"
  alt="99 percent code coverage"
/>

Every source file but one is covered at 100%. We can have great confidence in our
application, and safely refactor the code knowing that we have a robust set of
end-to-end tests.

If possible, we advise implementing
[visual testing](/app/tooling/visual-testing) in addition to Cypress
functional tests to avoid CSS and visual regressions.

## Combining code coverage from parallel tests

If you execute Cypress tests in
[parallel](/cloud/features/smart-orchestration/parallelization), each machine ends
up with a code coverage report that only shows a portion of the code exercised.
Typically an external code coverage service would merge such partial reports for
you. If you do want to merge the reports yourself:

- on every machine running Cypress tests, copy the produced code coverage report
  into a common folder under a unique name to avoid overwriting it
- after all E2E tests finish, combine the reports yourself using the `nyc merge`
  command

You can find an example of merging partial reports in our
[cypress-io/cypress-example-conduit-app](https://github.com/cypress-io/cypress-example-conduit-app)

## E2E and unit code coverage

Let's look at the one file that has a "missed" line. It is the
`src/selectors/index.js` file shown below.

<DocsImage
  src="/img/app/code-coverage/selectors.png"
  alt="Selectors file with a line not covered by end-to-end tests"
/>

The source line not covered by the end-to-end tests shows an edge case NOT
reachable from the UI. Yet this switch case is definitely worth testing - at
least to avoid accidentally changing its behavior during future refactoring.

We can directly test this piece of code by importing the `getVisibleTodos`
function from the Cypress spec file. In essence we are using Cypress as a unit
testing tool (find more unit testing recipes
[here](https://github.com/cypress-io/cypress-example-recipes#unit-testing)).

Here is our test to confirm that the error is thrown.

```javascript
// cypress/e2e/selectors.cy.js
import { getVisibleTodos } from '../../src/selectors'

describe('getVisibleTodos', () => {
  it('throws an error for unknown visibility filter', () => {
    expect(() => {
      getVisibleTodos({
        todos: [],
        visibilityFilter: 'unknown-filter',
      })
    }).to.throw()
  })
})
```

The test passes, even if there is no web application visited.

<DocsImage
  src="/img/app/code-coverage/unit-test.png"
  alt="Unit test for selector"
/>

Previously we instrumented the application code (either using a build step or
inserting a plugin into the Babel pipeline). In the example above, we are NOT
loading an application, instead we are only running the test files by
themselves.

If we want to collect the code coverage from the unit tests, we need to
instrument the source code of _our spec files_. The simplest way to do this is
to use the same `.babelrc` with
[`babel-plugin-istanbul`](https://github.com/istanbuljs/babel-plugin-istanbul)
and tell the Cypress built-in bundler to use `.babelrc` when bundling specs. One
can use the
[`@cypress/code-coverage`](https://github.com/cypress-io/code-coverage) plugin
again to do this by adding the code below to the
[setupNodeEvents](/app/plugins/plugins-guide#Using-a-plugin) function.

:::cypress-config-plugin-example

```javascript
require('@cypress/code-coverage/task')(on, config)
// tell Cypress to use .babelrc file
// and instrument the specs files
// only the extra application files will be instrumented
// not the spec files themselves
on('file:preprocessor', require('@cypress/code-coverage/use-babelrc'))

return config
```

:::

For reference, the `.babelrc` file is shared between the example application and
the spec files, thus Cypress tests are transpiled the same way the application
code is transpiled.

```json
{
  "presets": ["@babel/preset-react"],
  "plugins": ["transform-class-properties", "istanbul"]
}
```

When we run Cypress with
[`babel-plugin-istanbul`](https://github.com/istanbuljs/babel-plugin-istanbul)
included and inspect the `window.__coverage__` object in the **spec iframe**, we
should see the coverage information for the application source files.

<DocsImage
  src="/img/app/code-coverage/code-coverage-in-unit-test.png"
  alt="Code coverage in the unit test"
/>

The code coverage information in unit tests and end-to-end tests has the same
format; the
[`@cypress/code-coverage`](https://github.com/cypress-io/code-coverage) plugin
automatically grabs both and saves the combined report. Thus we can see the code
coverage from the `cypress/e2e/selectors.cy.js` file after running the test.

<DocsImage
  src="/img/app/code-coverage/unit-test-coverage.png"
  alt="Selectors code coverage"
/>

Our unit test is hitting the line we could not reach from the end-to-end tests,
and if we execute all spec files - we will get 100% code coverage.

<DocsImage
  src="/img/app/code-coverage/100percent.png"
  alt="Full code coverage"
/>

## Full stack code coverage

A complex application might have a Node back end with its own complex logic.
From the front end web application, the calls to the API go through layers of
code. It would be nice to track what back end code has been exercised during
Cypress end-to-end tests.

Are our end-to-end tests that are so effective at covering the web application
code also covering the back end server code?

**Long story short: yes.** You can collect the code coverage from the back end,
and let the `@cypress/code-coverage` plugin merge it with the front end
coverage, creating a single full stack report.

:::info

The full source code for this section can be found in the
[cypress-io/cypress-example-conduit-app](https://github.com/cypress-io/cypress-example-conduit-app)
repository.

:::

You can run your Node server and instrument it using nyc on the fly. Instead of
the "normal" server start command, you can run the command
`npm run start:coverage` defined in the `package.json` like this:

```json
{
  "scripts": {
    "start": "node server",
    "start:coverage": "nyc --silent node server"
  }
}
```

In your server, insert another middleware from `@cypress/code-coverage`. If you
use an Express server, include `middleware/express`:

```javascript
const express = require('express')
const app = express()

require('@cypress/code-coverage/middleware/express')(app)
```

If your server uses hapi, include `middleware/hapi`

```javascript
if (global.__coverage__) {
  require('@cypress/code-coverage/middleware/hapi')(server)
}
```

**Tip:** you can conditionally register the endpoint only if there is a global
code coverage object, and you can
[exclude the middleware code from the coverage numbers](https://github.com/gotwarlost/istanbul/blob/master/ignoring-code-for-coverage.md):

```javascript
/* istanbul ignore next */
if (global.__coverage__) {
  require('@cypress/code-coverage/middleware/hapi')(server)
}
```

For any other server type, define a `GET /__coverage__` endpoint and return the
`global.__coverage__` object.

```javascript
if (global.__coverage__) {
  // handle "GET __coverage__" requests
  onRequest = (response) => {
    response.sendJSON({ coverage: global.__coverage__ })
  }
}
```

In order for the `@cypress/code-coverage` plugin to know that it should request
the back end coverage, add the new endpoint to the Cypress configuration
environment settings under `env.codeCoverage.url` key. For example, if the
application back end is running at port 3000 and we are using the default "GET
/**coverage**" endpoint, set the following:

:::cypress-config-example

```ts
{
  env: {
    codeCoverage: {
      url: 'http://localhost:3000/__coverage__'
    }
  }
}
```

:::

From now on, the front end code coverage collected during end-to-end tests will
be merged with the code coverage from the instrumented back end code and saved
in a single report. Here is an example report from the
[cypress-io/cypress-example-conduit-app](https://github.com/cypress-io/cypress-example-conduit-app)
example:

<DocsImage
  src="/img/app/code-coverage/full-coverage.png"
  alt="Combined code coverage report from front and back end code"
/>

You can explore the above combined full stack coverage report at the
[coveralls.io/github/cypress-io/cypress-example-conduit-app](https://coveralls.io/github/cypress-io/cypress-example-conduit-app)
dashboard. You can also find full stack code coverage in our
[RealWorld App](https://github.com/cypress-io/cypress-realworld-app).

Even if you only want to measure the back end code coverage Cypress can help.

## Videos

There is a series of videos we have recorded showing code coverage in Cypress

### How to instrument react-scripts web application for code coverage

<DocsVideo
  src="https://youtube.com/embed/edgeQZ8UpD0"
  title="How to instrument react-scripts web application for code coverage"
/>

### Get code coverage reports from Cypress tests

<DocsVideo
  src="https://youtube.com/embed/y8StkffYra0"
  title="Get code coverage reports from Cypress tests"
/>

### Excluding code from code coverage reports

<DocsVideo
  src="https://youtube.com/embed/DlceMpRpbAw"
  title="Excluding code from code coverage reports"
/>

### Check code coverage robustly using 3rd party tool

<DocsVideo
  src="https://youtube.com/embed/dwU5gUG2"
  title="Check code coverage robustly using 3rd party tool"
/>

### Adding code coverage badge to your project

<DocsVideo
  src="https://youtube.com/embed/bNVRxb-MKGo"
  title="Adding code coverage badge to your project"
/>

### Show code coverage in commit status check

<DocsVideo
  src="https://youtube.com/embed/AAl4HmJ3YuM"
  title="Show code coverage in commit status check"
/>

### Checking code coverage on pull request

<DocsVideo
  src="https://youtube.com/embed/9Eq_gIshK0o"
  title="Checking code coverage on pull request"
/>

## Examples

You can find full examples showing different code coverage setups in the
following repositories:

- [cypress-io/cypress-realworld-app](https://github.com/cypress-io/cypress-realworld-app)
  or RWA is a full stack example application that demonstrates **best practices
  and scalable strategies with Cypress in practical and realistic scenarios**.
  The RWA achieves full code coverage with end-to-end tests
  [across multiple browsers](/app/guides/cross-browser-testing) and
  [device sizes](/api/commands/viewport).
- [lluia/cypress-typescript-coverage-example](https://github.com/lluia/cypress-typescript-coverage-example)
  shows coverage for a React App that uses TypeScript.
- [ericorruption/cypress-code-coverage-typescript-webpack-ts-loader](https://github.com/ericorruption/cypress-code-coverage-typescript-webpack-ts-loader)
  shows how to collect coverage for a TypeScript + webpack project using
  `ts-loader` instead of `babel-loader`.

Find the full list of examples linked in
[cypress-io/code-coverage#external-examples](https://github.com/cypress-io/code-coverage#external-examples).

## See also

- [UI Coverage](/ui-coverage/get-started/introduction)
- The official
  [@cypress/code-coverage](https://github.com/cypress-io/code-coverage) plugin
